Current limiting fuse with improved fuse elements

ABSTRACT

A cartridge type current limiting electrical fuse including at least one fuse element. The element is made from a length of fusible material which is tapered substantially along its legth to a minimum central taper point or portion and then widened again, or which is varied in discrete steps along its length. The fusible material is then wound into a helical or spring-like shape. However, the effective diameter of the resulting springlike solenoid or helically wound coil is made to vary in accordance with the variation in the diameter of the fuse material. The smaller the diameter of the fuse material at any point along its length, the smaller is the diameter of the corresponding solenoid or coil at that point or portion. If the length of fusible material varies continuously between alternating maximum and minimum cross-sectional areas, the finished or overall helical coil or fuse element solenoid will vary continuously to form corresponding maximum and minimum alternating solenoid diameters. If the cross-sectional area of the fuse material varies discretely such as in steps, as would be the case if sections of fusible material of varying diameters or size were joined end-to-end, the corresponding solenoid or helical coil formed will also vary discretely. The size of the diameter for any section of wire depends upon the spring constant or spring rate developed in the solenoid or coil of fusible material. The spring rate is kept preferably substantially constant for any section of fusible wire regardless of the wire&#39;&#39;s diameter by adjusting the diameter of the fuse to a predetermined value for the diameter of the wire. Consequently, a spring-like fuse element is ultimately formed having a generally constant spring-rate along substantially its entire length. If the spring shaped fuse element is then either expanded or contracted longitudinally within a reasonable limit generally corresponding to Hooke&#39;&#39;s Law, the pitch of the helical spring element will remain constant along the length of the spring although naturally it will change for each increment the spring is expanded or contracted.

United States Patent Cameron [451 July 17, 1973 CURRENT LIMITING FUSEWITH IMPROVED FUSE ELEMENTS [75] Inventor: Frank L. Cameron, Irwin, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Jan. 21, 1972 [21] Appl. No.: 219,712

Primary Examiner-Roy N. Envall, Jr. Attorney-A. T. Stratton 37 ABSTRACTA cartridge type current limiting electrical fuse including at least onefuse element. The element is made from a length of fusible materialwhich is tapered substantially along its legth to a minimum centraltaper point or portion and then widened again, or which is varied indiscrete steps along its length. The fusible material is then wound intoa helical or spring-like shape. However, the effective diameter of theresulting spring-like solenoid 0r helically wound coil is made to varyin accordance with the variation in the diameter of the fuse material.The smaller the diameter of the fuse material at any point along itslength, the smaller is the diameter of the corresponding solenoid orcoil at that point or portion. If the length of fusible material variescontinuously between alternating maximum and minimum cross-sectionalareas, the finished or overall helical coil or fuse element solenoidwill vary continuously to form corresponding maximum and minimumalternating solenoid diameters. If the cross-sectional area of the fusematerial varies discretely such as in steps, as would be the case ifsections of fusible material of varying diameters or size were joinedend-to-end, the corresponding solenoid or helical coil formed will alsovary discretely. The size of the diameter for any section of wiredepends upon the spring constant or spring rate developed in thesolenoid or coil of fusible material. The spring rate is kept preferablysubstantially constant for any section of fusible wire regardless of thewires diameter by adjusting the diameter of the fuse to a predeterminedvalue for the diameter of the wire. Consequently, a spring-like fuseelement is ultimately formed having a generally constant spring-ratealong substan tially its entire length. lf the spring shaped fuseelement is then either expanded or contracted longitudinally within areasonable limit generally corresponding to Hookes Law, the pitch of thehelical spring element will remain constant along the length of thespring although naturally it will change for each increment the springis expanded or contracted.

10 Claims, 13 Drawing Figures mmmnm 3.147. 041

SHiH 1 [IF 3 FIG] PRIOR ART FIG2 PRIOR ART 1 CURRENT LIMITING FUSE WITHIMPROVED FUSE ELEMENTS BACKGROUND OF THE INVENTION This inventionrelates to current limiting fuses in general and in particular tohelical current limiting fuse links. A current limiting fuse element maybe formed with its cross section along its length varied to cause thefuse element to form multiple arcs at predetermined points along itslength as the melting temperature of the fuse material is reached suchas disclosed in US. Pat. No. 2,157,907 issued to K. H. Lohausen May 9,1939. This construction is employed to produce a plurality of voltagedroping arcs which tend to limit the amount of current flowing throughthe fuse element during overload conditions. Occasionally in highvoltage applications fuse elements of this type are required to be verylong in addition to being tapered or stepped in size; however, it isimpractical for reasons of overall size in such applications to employ asingle length of straight fusible material in a cartridge type fuse. Onesolution has been to wind the fusible material into a helical coil sothat the effective axial length of the fuse element is made suitable formounting in a particular size of cartridge type fuse while the effectiveoverall length of the fuse element is sufficient for a particularvoltage application.

In the past, helically wound tapered or stepped fuse elements have beenmounted within cartridge type fuses on mandrels or supports which aremade of electrically insulating material. In such a construction, thefuse element is pre-assembled upon the support or mandrel, and themandrel and the accompanying fuse element are assembled in a cartridge,housing. and surrounded by or embedded in a pulverulent arcquenchingmaterial, such as quartz sand, during final assembly. However, the useof fuse element supports or mandrels has proved to be expensive, timeconsuming and electrically poorer in certain applications. Consequentlyit has been proposed that lengths offusible material containing steppedor tapered portions be prewound and removed from the mandrel upon whichthey were wound and stretched at both ends to be conveniently assembledin a cartridge type fuse. Unfortunately, regardless of how uniformly thefuse element may have been initially wound as it is stretched formounting in the cartridge type fuse, the pitch of adjacent sections offuse spirals would change as a function of the varying diameter or sizeof the fusible material from which the fuse element is formed. Theportions of the fusible material with very small cross section tend toexpandto a much greater degree than the ones with larger cross sectionsafter removal from the fabrication mandrel on which the fuse element isinitially wound. Consequently, it was found that although certainadvantages may be achieved by eliminating a mandrel support for helicalfuse elements, other disadvantages are encountered due to the fact thatthe fulgurite formed around different portions of the blown fuseelement, may expand onto or overlap an adjoining closely spaced turn orhelical portion of the fuse element in such a manner that theelectrically insulating properties of the fulgurite or fused quartz sandmaterial is not sufficient to electrically insulate the overlappedhelical portion from remaining parts of the spent or melted fuseelement. This may result in a short circuit or electrical conductivitybetween the remaining parts of the spent fuse element and what is leftof a helical portion of the fuse element. This short circuit may beshort lived or relatively permanent and may have the effect of changingthe interrupting characteristics of the instantaneously melting fuseelement or providing a permanent short circuit between what should beelectrically insulated portions of an interrupted electrical circuit.Therefore, it is desirable to eliminate the need for mandrels for fuseelements of relatively long lengths which may be formed into the shapeof a helical coil having a substantially constant minimum pitch betweenhelical sections or portions.

SUMMARY OF THE INVENTION In accordance with the invention, a fusibleelement for cartridge type current limiting fuse is formed by windingvariable cross-section fusible material into a helical coil. Theeffective diameter or size of the overall fuse element varies along theaxial length of the fuse element as a predetermined function of thediameter of the fusible material. For example, a long length of fusiblematerial which is in the form of tapered circular cross-section wirehaving a plurality of narrow neck portions of narrow cross-sectionalportions where the fuse element will blow or melt initially is woundinto a solenoid-like or helical coil fuse element having residualspringiness. Those parts of the solenoidor coil corresponding to thesmaller cross-sections of the wire are more tightly wound or in otherwords wound with a smaller diameter to form the .coil or solenoid. Thoseparts of the larger diameter fusible wire near the ends of the coil orsolenoid are wound with the largest effective diameter. The ultimate aimin winding a fuse element of fusible wire or fusible material in thismanner is to provide a helically coiled fuse element which may beexpanded or compressed like an accordion but in such a manner that thepitch between adjacent sections of each contributing member of thespiral is always substantially the same. When the fuse element is fullyexpanded or tensioned, the axial spacing between each adjacent turn orhelical sections is relatively large but nevertheless substantiallyequal for every helical portion or section of the overall fuse element.Correspondingly, as the winding is compressed, the spacing between wiresor between adjacent sections or turns of the overall coil or solenoidbecomes relatively smaller but again is substantially equal for thedifferent adjacent sections or portions of the overall fuse element. Theimportance of this construction can be seen from the fact that when thefuse element is caused to melt or blow due to an overload currentwhichcauses the fuse element to heat the adjacent pulverulentarc-quenching material fuses into a cohesive conglomerate or fulgurite.Depending upon the voltage impressed between opposite separated ends ofthe fuse element portions, the fuse element may burn awayor melt in aplurality of current limiting sections which may generally correspond tothe number of previously mentioned thin necks or reduced cross-sectionalportions along the length of the fuse element. As adjacent ends of eachof these neck portions begin to burn away from each other due to thefusing action of the arc struck between them, a point is reached atwhich the distance between the ends of the blown portion is so largethat the interposed fulgurite acts as a sufficient dielectric to preventfurther electrical conduction between burned away ends of the remainderof the fuse element. This may happen substantially instantaneouslydepending upon the overload current since an entire helical portion of afuse element may burn away within a fraction of a second. However, theexpanding, heated fulgurite which has limited electrical conductingproperties may overlap to make electrical contact with the next adjacenthelical section or turn of the fuse element. The distance between oneend of the melted fusible material and the next adjacent helical sectionmay be considerably smaller than the distance between the two remainingend sections or end pieces of the fusible wire or material. A shortcircuit or electrically conducting path may then exist between thehigher voltage end of the burned away fuse element and the next adjacentturn or helical portion which is electrically connected to the lowervoltage end of the burned away fuse element. In some instances, thisphenomena or operation may have the effect of modifying or distortingthe interrupting characteristics of the fuse element or in another sensecausing a permanent short circuit between supposedly interrupted ends ofan electrical fuse if not prevented by the teachings of this invention.

Fuse elements similar to those previously described may be formed inaccordance with the invention by using a length of fusible materialcomprising adjacent or serially connected sections of fusible wire whichmay be circular and which may have different diameters. Those wireshaving the smaller diameters or smaller cross-sectional areas are ofcourse more likely to burn away or blow when an overload currentcondition exists in the cartridge fuse. A fuse element embodying theteachings ofthe invention may also be formed by winding the fusible wireor material in such a manner thatthe radius or diameter of the resultantsolenoid or coil or spring-like fuse element varies in discrete steps.Those fuse elements formed from placing end-to-end sections of wire eachhaving a different but substantially constant diameter may be pre-woundon a manufacturing mandrel in such a way that those sections of wirehaving relatively large circular cross-sectional areas will be wound inrelatively larger diameter portions of the overall solenoid or coil andthose sections of wire having relatively smaller diameters will beformed into those parts of the solenoid having relatively smallersolenoid diameters. The reason for this is to provide a substantiallyconstant spring pressure or spring constant along the entire length ofthe solenoid or helical fuse element. A substantially uniform springrate allows the wire to be expanded, for example, to a length suitablefor mounting in a cartridge type fuse and concurrently having a constantpitch which provides adjacent sections or coils of the overall coilwhich are sufficiently spaced from other turns or sections so that aburned out or fused portion of the wire will not be electricallyconnected by a section of fulgurite to form an electrically conductingpath or short circuit circuit between adjacent helical sections or turnsof the overall fuse clement.

The teachings of this invention also include the use of uniquemanufacturing or forming mandrels and a process for manufacturing thepreviously mentioned solenoids or spring-like coils of fuse elements. Inthe instance of a tapered fuse wire, assuming for example that the wireis tapered so that it is wider or larger at the ends than at the middle,a mandrel which is correspondingly wider or broader at the ends than inthe middle may be formed by joining two sections of mandrel together atthe smaller inner area in any convenient fashion such as by threadingone end of one mandrel and tapping a hole in the corresponding end ofthe other mandrel and screwing one mandrel into the other. The taperedfuse wire can then be wound on the mandrel forming a coil havinggradually smaller diameter as the wire advances toward the middle of themandrel and then a larger diameter as the wire advances away from themiddle towards the other end of the mandrel. The mandrel can then beunscrewed and pulled out from the formed spiral or solenoid withoutdistorting or in any way damaging the formed solenoid or coil. Similarlythe same method can be employed with a stepped mandrel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a prior art fuse elementformed in the shape of a solenoid or helical coil and having a melted orblown portion;

FIG. 2 shows a longitudinal section of a prior art tapered fuse elementor wire with a blown portion;

FIG. 3 shows a prior art fuse element assembly with parallel fuseelements or wires wound on a supporting member or mandrel;

FIG. 4 shows a prior art cartridge fuse structure with parallel fuseelements mounted in the associated housing without the benefit of amandrel or supporting member;

FIG. 5 shows a prior art fuse element including stepped or discretesections of fuse wires joined end-toend and a corresponding solenoid orfuse element coil that may be formed from such a fuse element;

FIG. 6 shows a stepped fuse wire similar to that shown in FIG. 5 butformed into a substantially constant pitch helically coiled fuse elementin accordance with the invention;

FIG. 7 shows a section or length of tapered fuse wire and acorresponding prior art solenoid or coil type fuse element which may beformed from it;

FIG. 8 shows a second fuse element in accordance with the inventionwhich may be formed from the ta-' pered fuse wire shown in FIG. 7 as asubstantially constant pitch-solenoid or coil;

FIG. 9 shows a cartridge type fuse similar to the one shown in FIG. 4but with a substantially constant pitch solenoid or coil fuse elementformed from fusible material of discretely varying size;

FIG. 10 shows a cartridge fuse similar to the one shown in FIG. 4 with asubstantially constant pitch solenoid or coil fuse element formed fromfusible material of continually varying or tapered diameter or size inaccordance with the invention;

FIG. 11 shows a mandrel for forming a substantially constant pitchhelically wound fuse element of discretely varying size in accordancewith the invention;

FIG. 12 shows a mandrel for forming a constant pitch helicallycontinuously varying spiral fuse section in accordance with theinvention; and

FIG. 13 shows a helical describing vector system.

DETAILED DESCRIPTION OF THE DRAWING Referring now to the drawing andFIG. 1 in particular, a prior art generally helically wound fuse element10 is shown which is suitable for mounting in a current limitingcartridge type fuse. The fuse element 10 comprises a length of fusiblematerial 12 wound into the shape of a resilient or spring chargedhelical coil having a diameter D. The size of the fusible material 12may be of the tapered or stepped variety. The fuse element shown istypical of many known helically wound fuse elements, during interruptionit may have a relatively high voltage V1 applied at end or terminal 16and a different voltage V2 applied at another end or terminal 14. Acurrent I is shown flowing through a portion of the length of fusiblematerial 12. Current I may be of sufficient magnitude to cause fusiblematerial 12 to flow or melt at a predetermined weakened point or portionin the fusible 12 between points or fused end sections 20 and 24. Whenthe fusible material melts or blows, a high voltage are is nearlyinstantaneously established between point or arc anode 20 and arecathode 24 to limit the rate of magnitude of current I. The fusiblematerial 12 is normally surrounded by a pulverulent arc quenchingmaterial 27 and as the heat of the arc established between points orends 20 and 24 melts or fires thearc-quenching material 27, a fulguriteor conglomerated mass of arc-quenching material 28 is formed. The. arcmay cause the fusible material 12 to burn backwards such that points orends 20 and 24, although shown in one instant of time, effectivelyprogress or move away from each other as the arc between them consumesthe fusible material forming fuse wire 12. The arc will cease or beinterrupted when electrical point 20, which may be at high electricalpotential V1, is sufficiently burned back or removed a distance L2 fromelectrical point 24, which may be maintained at a significantly lowerelectrical potential V2, to create an insulating gap filled with fusedpulverulent material 28. However, the fulgurite material 28 which mayhave expanded outwardly and which may have sufficient electricalconductivity such that the distance represented by Ll between electricalpoints 20 and 32, which may also be maintained as voltages V1 and V2respectively, is insufficient to prevent the continued arcing or flow ofcurrent I along path L1. In a short term sense or in the instant afterthe fuse element has begun to melt the short circuiting of current Ithrough path L1 may cause a drastic change in the current limitingcharacteristics of'fuse element 10. In a long term or long time sense,the availability of the short circuit or conducting path Ll between thepoints and 32 will allow current I to flow between terminals 16 and 14even though fuse element 10 ostensibly has blown or opened between theburned away points 20 and 24.

Referring now to FIG. 2, a view ofa prior art fuse element 10. includingelectrically conducting fusible material 12 similar to fusible material12 shown in FIG.

1 is depicted in the vicinity of a current limiting are terial 28 themelted ends ofthe fusible material 12' are separated by distance L2. Avoltage or potential V1 exists at point or region 20 and the voltage orpotential V2 exists at point or region 24'. The electrically conductingproperties of the fulgurite 28 may be sufficient in this geometricconfiguration to prevent further current flow between points 20' and 24.This is the way a fuse normally provides an insulating gap in theelectrical curcuit in which it is connected.

By referring once again to. FIG. 1, it can be shown that if the distanceLl can be made larger than the distance L2, which is the maximumdistance over which an arc may be sustained between points 20 and 24 inthe presence of a predetermined potential difference, an additional arcalong line Ll between points 20 and 32 will'not be possible. The minimumdistance between point 20 and anadjacent point such as 32 in the nextturn or helical portion is represented by the distance S; S being themeasure of the pitch of the winding of the coil of fuse element 10.

Referring now to FIG. 3, a prior art cartridge fuse structure 40 isshown having electrically conducting end sections or ferrules 44 and 46with contacting members 50 and 48 respectively. Tie-down or braid tiepoints or end terminals 52 and 54 are shown mounted adjacent to ferrulesor end caps 46 and 44 respectively. A mandrel or electrically insulatingsupporting member 56 is disposed between end portions or end caps 44 and46. An electrically insulating cylindrical or tubular casing 43surrounds the mandrel56 and an interposed arc quenching material 57. Asillustrated there are five parallel fuse elements or fuse wires spacedfrom one and connected between the common points 52 and 54 of fusesection 40 as indicated by fusible conducting wires or elements 12a,12b, 12c, 12d and 12e. The minimum distance between any two wires S canbe quite easily maintained at a value sufficient to prevent flashoverbetween adjacent turns or helical sections 53 and 55 for example. Thefixed distance S is maintained by relatively rigidly winding the wires12a, 12b and 12c and 12d and l2e on the supporting mandrel 56.

Referring now to FIG. 4, a prior art fuse structure similar to the oneshown in FIG. 3 isdepicted. End sections or ferrules 44 and 46' areshown'having a cylindrical electrically insulating main housing 43disposed between and supporting them. A pulverulent arc quenchingmaterial 57 such as quartz or silica sand is enclosed by cylinder 43'and end sections 46' and 44'. Spring loaded electrically conductingplunger 48' is also provided. Helically wound or charged fuse elementsof fusible material 60 and 62 are mounted within casing 43'. Helicalcoil or solenoid 60 is supported at ends 64 and 66 and solenoid orhelically wound fuse element 62 is supported at ends 68 and 70. Thehelically wound fuse elements 60 and 62 have been prefabricated on amandrel, removed from the mandrel and stretched spring loaded or chargedto be placed or assembled in fuse structure 56. Pitch S varies along theentire axial length of both solenoids 60 and 62. This is because theoverall diameter of the spring-like winding generally constant along itslength and, because the mandrel upon which the winding waspre-fabricated has 'a uniform diameter. However in most circumstancesthe diameter of the fusible material from which the fuse elements areformed is varied either continu ously or discretely in steps in order toestablish points or areas where arcs may begin upon the heating of thefuse elements'60 and 62 due to overcurrent. But this causes the smallersections of fuses 60 and 62 of cross sectional area to be pulled apartor elongated to a larger extent by the tensioning or stretching of thespring-like fuse elements 60 and 62, than the portions of said fuseelements where thee may be more crosssectional area in the fusiblematerial. Consequently, a minimum pre-determined spacing such as spacingS as shown in FIG. 3 cannot be maintained and an arc-over betweenadjacent spiral sections of either fuse piece 60 or 62 may occursimilarly to that which was described with respect to FIG. 1. Referringnow to FIG. 5, a prior art fuse element or stepped fusible material 72is shown. Fusible material 72 comprises, in this example, three fusewires of generally circular cross-section abutted or joined end-toend insome convenient fashion such as brazing. Wire 74, having a diameter d1is abutted at point 80 to fuse wire 76 having diameter d2 and fuse wire76 is abutted at point 82 to fusible material 78 having diameter d3.When the composite length of fusible material 72 is wound into the shapeof a helical coil which has a single overall diameter DI, the pitch ofadjacent sections of spiral may vary depending upon the strength orcrosssectional size of the fuse material making up each particular partof the winding. For example, the pitch S1 existing between points 90 and92 of fusible material 74 may be greater than the pitch S2 existingbetween points 94 and 96 of the fusible material 76 which, in turn, maybe different than the pitch S3 existing between points 101 and 102 offusible material 78. It will be noted that the solenoid or spring-likefuse element 73 has a common centerline or longitudinal axis 84.

Referring now to FIG. 6, the disclosed invention is illustrated in afuse element 73'. In ths construction, the length of fusible wire 72 aswas shown in FIG. is once again wound into a helically shaped fuseelement 73. However, the pitch between adjacent sections of each fuseelement is mantained at a substantially constant or predetermineddistance which may be S4 as shown in this case. In other words, thedistance between adjacent corresponding points of fusible material orwire 74, as shown by S4, between points 90' and 92' is generally thesame as the distance S4 which may exist between corresponding points orturns 94' and 96 of fusible material 76. This also applies for thedistance S4 between points 100' and 102' of the fusible material 76. Acommon centerline or longitudinal axis 84' exists for the helical fuseelement. An important difference between fuse elements 73 and 73' isthat the overall diameter of the formed helix or fuse element 73 variesas a function of the thickness or diameter of the fusible material orwire that is being wound into the helix shape. Consequently, wire 74having a diameter d1 is formed into a helical fuse element portionhaving a helical diameter D4. Likewise, fusible wire 76 having adiameter d2 is formed into a helical portion having a helical diameterD2 and finally fusible wire 78 having a diameter d3 is formed into ahelical fuse element portion having a diameter D3. Consequently, it canbe seen that the diameter of the helical section of stepped ordiscretely varying fuse material is made substantially proportional tothe actual diameter of the wire forming the particular portion of theoverall helical fuse element. This fuse element construction provides agenerally constant spring rate along the length of the solenoid orhelical section 73'. As a result, a fuse element can be formed withgreat accuracy so that the distance between adjacent corresponding turnsor points such as 94' and 96 in the same longitudinal plane (providedthat longitudinal plane contains centerline 84') will be the same forany degree of extension or flexing of the helical fuse element 73. Ifhelical fuse element 73 is compressed in an accordion-like manner, thedistance between adjacent corresponding points of the helical fuseelement will become smaller but will nevertheless remain substantiallyuniform or constant along the entire axial length of the helical fuseelement. The same result applies for an extension of the spring-likefuse element 73'.

Referring now to FIG. 7, a prior art fuse element including a length ofcontinuously tapered fusible material 104 is shown. Fusible material 104is circular and has a relatively larger diameter at its ends 108 and 110and a relatively smaller diameter in the middle 106. The variationsbetween points 108 106 and 110 are continuous rather than discrete, aswas shown in FIG. 5 for the fusible materials employed. Consequently,the helical section or spring-like section 105 of silver com positionfusible material when stretched or extended will provide varying spaced,distances or gaps between adjacent corresponding sections or turns ofthe helical section 105. For example distance S5 between points 114 and116 on the helix 105 is shorter than the distance S6 between points 118and 120. This is true be cause the helically wound fusible wire in thevicinity of the narrower cross-sectional area at 106 is more easilystretched in a longitudinal or axial direction than the fusible wire inthe vicinity of the relatively larger size end sections 108 and 110. Ifthe distance S5, as an example, were smaller than the minimum breakdownvoltage path through the formed fulgurite between points 116 and 114, ashort circuit or conducting path may result between these points eventhough the fuse element had blown elsewhere either in the vicinity ofpoint 116 or in the vicinity of point 114. Referring now to FIG. 8, tocompensate for the problem shown in FIG. 7, the fusible wire 104 inaccordance with the invention may be wound into a helically shaped coil105' having a varying overall diameter. For example, the diameter of thehelix 105' in the vicinity of the relatively larger cross-sectionalareas of wire 106 near ends 108 and 110 is shown as D6 which is largerthan the overall diameter D7 of the helix or fuse element portion shownin the vicinity 104 of the relatively smaller crosssectional portion ofthe wire 106. The effect of this construction is to maintainsubstantially uniform spacings S7 between adjacent corresponding turnsor sections of the helical fuse elemlent 105' which are equal for anygiven extension of the helix or fuse element 105 For example, thedistance or spacing S7 between points 114' and 116 is generally equal tothe distance S7 between the points 118' and 120' even though thecross-sectional area of the fusible material in wire 106 is larger inthe vicinity of points 114' and 116' than the cross-sectional area ofthe material in the wire 106 in the vicinity of the ends 118 and 120.

Referring now to FIG. 9, a fuse structure 56' generally similar to thefuse structure 56 in FIG. 4 is shown. There are end caps 44" and 46" anda spring loaded conducting plunger 48". A pulverulent arc quenchingmaterial, such as silica sand 57" is enclosed within an electricallyinsulating cylinder or housing 43" and around the helical fuse elements73T" and 73B". Fuse element 73T" is stretched and connected betweenelectrical contact points or terminals 64 and 66 which are connected toelectrical end caps or ferrules 46" and 44" respectively. In a similarmanner, helical fuse element 738" is connected at the ends 68 and 70 tothe end ferrules or caps 46" and 44" respectively. It

will be noted that the helical fuse elements 73'1" and 73B" are of thediscrete or stepped variety with respect to size as depicted in FIG. 6.The helical electrical fuse element of 7ST" for example is divided infive easily identifiable discrete regions or portions each one having adifferent solenoid diameter associated therewith. Regions 124 and 132have the relatively largest diameters; regions 126v and 130 have smallerdiameters and region 128 has the smallest diameter. It is important tonote that thepitch or spacing between adjacent corresponding points orturns of each of the fuse elements 73T" and 73B" is substantiallyuniform-or equal along the axial length of each of said fuse elements.

Referring now to FIG. 10, a similar current limiting fuse structure 56"is-shown having a plurality of helical fuse elements 105T" and 1058"with continuously varying solenoid diameters. Helical fuse element 105T"is suspended orconnected between end points or.terminals 64"0 and 66"and helical fuse element 1058" connected between end points 68" and 70'.It will be noted that the pitch of each of the helical fuse elemets T"and 10513" is thesame across the entire length of said helical fuseelements. However the amplitude or the diameter of the overall portionsof helical section vary in proportion to the diameter of the fusiblewire forming the helical portion at any point. Consequently, thediameter of the helical portion near, 105" for example, points 134 and136 is large because the diameter of the wire is relatively largerinthose areas and the diameter of the helical portion in the region ofpoint 133 is relatively smallerbecause the diameter or size of the wireis tapered or smaller in that area. Referring now to FIG. 11, amanufacturing mandrel is shown for prefabricating or forming a variablediameter fuse element helix. In this case, a stepped mandrel has leftsection 150L and a right section 150R which may be joined by thescrewing or turning of a threaded member 156 into a taped hole 154 ofsection 150R. The discrete steps are shown at points 152. Referring nowto FIG. 12 a similar manufacturing mandrel 160 for pre-fabricating orforming fuse elements from continously varying or tapered fusible wireis shown. Mandrel 160 may have left section 160L with a threadedprotrusion 166 and a right section 160R with a tapped hole 164. Section160L is screwed into the hole 164 in section 160R. A continuous taperresults which decreases towards the middle is shown along the surface162.

Referring now to FIG. 13, a means for theoretically showing thegeneration of the shape of a helical fuse element is shown. A centerlineor longitudinal axis 170 is established between end points 111 and 172.A rotatable or revolving vector V is mounted at one of its ends 173 tothe imaginary centerline or axis 170 and is capable of rotating aboutcenterline 170 with an angular velocity (O). The other end 174 of vectorV traces the path of the helical coil. Vector V is capable of beingmoved with the velocity (v) from point 171 to 172 or vice versa. As itmoves longitudinally the vector V rotates angularly about centerline 170with an angular velocity Q. The length of vector V is L(d), where d isthe diameter of the fuse wire. L(d) is variable such that the distancebetween hinge .point or pivot point 173 and end point 174 of vector Vmay vary as vector V is moved longitudinally and rotatably along line170. The

distance between points 171 and 172 is I. As vector V is moved point 14traces the path of the helix.

It is to be understood that a generally circular type helix iscontemplated in practicing the disclosed invention; however, helicalcoils of oblong cross-sections or elliptical cross sections may beprovided where desired. It is also to be understood that the discretetype variable diameter wire may have many discrete sections and thecorresponding helical section will reflect the. corresponding size ofthe wire along the length of the fuse material. It is also to beunderstood that the tapered type fuse helical section may havealternating large tapers and small tapers providing multiple are overpoints for the fuse material and correspondingly the helix form may havealternating large and small diameters along its length. It is to beunderstood that any type of fusible material may be used which issuitable for fusing or melting under overload conditions such as silveror silver alloys. It is also to be understood that the cross section ofthe wire itself may vary along its entire lengthand need not necessarilybe circular and it is also to be understood that the mandrels as shownin FIGS. 11 and 12 are only aids in forming the helical sectionsdisclosed and need not necessarily be the means for fabricating thehelcal sections. The mandrels sections may be joined in any convenientmanner. The helix shape may be joined inany convenient manner. The helixshape may vary continuously and is not limited to a tapered or steppedhelix shape. The voltages V1 and V2 may be of any value and V2 may behigher thanVl.

The apparatus embodying the teachings of this invention have manyimportant advantages. For example one advantage lies in the fact that ahelical fuse element may be provided without the use of a supportingmandrel which would therewise be mounted within a cartridge type fuse. Asubstantially uniform or constant minimum pitch or distance or spacingbetween adjacent corresponding portions of a fuse element is provided sothat as the fuse element blows or is melted a short circuit orelectrically conducting path is not provided between adjacent fuseelements due to proximity and the formation of fulgurite material whichmay temporarily act as a conducting medium. Since a mandrel is notrequired to be used to maintain the exact spacing between adjacentspiral sections of fuse elements, the heat dissipated during the fusingor melting process may be more easily absorbed by the arc quenchinganheat absorbing pulverulent material such as silica or quartz sand. Inaddition since the diameters of the respective sections of a length offusible material may vary, when forming the helical fuse element, thefuse element may be compressed or stretched to any reasonable length,not exceeding Hookes Law, while maintaining relatively constant distancebetween each adjacent section of the fuse element for the compressing orstretching of the helical section. This aids in the convenient assemblyand manufacture of the overall fuse structure. It is also to beunderstood that in some instances the fuse elements may be prefabricatedon mandrels which are separable after pre-fabrication in such a mannerthat the mandrels can be removed with out destroying the shape of theformed helical fuse section. Another advantage lies in the fact thatsince the tension between adjacent sections of fuse material is retainedat a constant spring rate or tension rate the likelihood of a tearing,breaking or failure of the fuse element for reasons other than the flowof overcurrent may be minimized. Another advantage lies in the fact thatif the fuse element is pre-stressed or charged at a certain pitchdiameter when the fuse element blows there is less tendency for randomlongitudinal oscillation to occur among adjacent isolated parts of theblown helical element during the interrupting interval which might causeunusual and unpredicatable interrupting characteristics. Anotheradvantage lies in the fact that the use of a helical fuse element asdisclosed provides an easy way to mount long lengths of fusible wirewithin a cartridge type fuse configuration.

I claim as my invention:

1. A current limiting fuse structure comprising a fuse element includingfusible material having the characteristic shape of a winding, saidfusible material having a generally variable cross-sectional area, saidwinding having a generally central longitudinal axis, the shape of saidwinding relative to said axis being generally defined by the locus ofpoints formed by the end ofa movable vector, said vector being generallyterminated at said end by said fusible material and at the other end bysaid longitudinal axis, said vector being oriented perpendicular to saidlongitudinal axis, said locus of points being described by moving saidvector longitudinally along said axis at a fixed rate and concurrentlyrotating said vector at a fixed angular rate about said longitudinalaxis, the length of said vector at any time being related to said crosssectional area of said fusible material at any point along saidlongitudinal axis, said winding having two ends, said fuse structurehaving two terminals, said terminals being connected to said ends ofsaid fuse element, the longitudinal components of distance betweenadjacent sections of the winding as measured in a plane including saidlongitudinal axis being substantially equal, said winding beingsupported substantially only at said ends.

2. A current limiting fuse structure comprising a fuse element includingfusible material having the characteristic shape of a helical coil, saidfusible material having a generally variable cross-sectional area, saidhelical coil having a central longitudinal axis, a variable radius andtwo ends, said radius of said helical coil at any point being related tosaid cross-sectional area of said fusible material at that point, thelongitudinal components of distance between correspoonding portions ofsaid helical coil as measured in a plane including said longitudinalaxis being substantially equal, said distance being dependent on theaxial length of said helical coil within a predetermined range of axiallengths, said coil being supported substantially only at said two ends.

3. The combination as claimed in claim 2 wherein said cross-sectionalarea of said fuse material varies generally discretely in steps, saidvariable radius correspondingly varying discretely in steps.

4. The combination as claimed in claim 3 where in said cross-sectionalarea of said fuse material is generally circular.

5. The combination as claimed in claim 2 wherein said cross-sectionalarea of said fusible material is generally circular, said circularfusible material having a radius wherein said latter-mentioned radiusvaries substantiallyjinearly along the length of said fuse material,said radius of said helical coil correspondingly varying substantiallylinearly.

6. A current limiting fue structure comprising a fuse element,electrically conducting end pieces, a dielectric container disposed toenclose said fuse element, said container being disposed between saidend pieces, said fuse element comprising resilient fusible materialhaving the characteristic shape of a helical coil, said fusible materialhaving a generally variable crosssectional area, said helical coilhaving a variable radius, and a longitudinal central axis, said fusiblematerial having two ends connected to said electrically conducting endpieces, said radius of said helical coil being related to saidcross-sectional area of said fuse material such that the longitudinalseparations between adjacent portions of said helical coil, as measuredin a plane containing said longitudinal axis, are substantially equal,

regardless of the axial length of said helical coil within predeterminedlimits, said helical coil being supported only at said ends andprimarily by said end pieces.

7. The combinationas claimed in claim 6 wherein said cross-sectionalarea of said fuse material varies generally discretely in steps, saidvariable radius correspoindingly varying discretely in steps.

8. The combination as claimed in claim 7 wherein said cross-sectionalarea of said fuse material is generally circular.

9. The combination as claimed in claim 6 wherein said corss-sectionalarea of said fusible material is generally circular, said circular fusematerial having a radius wherein said latter-mentioned radius variessubstantially linearly along the length of said fuse material, saidradius of said helical coil correspoindingly varying substantiallylinearly.

10. The combination as claimed in claim 9 wherein pulverulent arcquenching material is disposed around said fuse element and containedwithin said dielectric cartridge.

1. A current limiting fuse structure comprising a fuse element includingfusible material having the characteristic shape of a winding, saidfusible material having a generally variable cross-sectional area, saidwinding having a generally central longitudinal axis, the shape of saidwinding relative to said axis being generally defined by the locus ofpoints formed by the end of a movable vector, said vector beinggenerally terminated at said end by said fusible material and at theotHer end by said longitudinal axis, said vector being orientedperpendicular to said longitudinal axis, said locus of points beingdescribed by moving said vector longitudinally along said axis at afixed rate and concurrently rotating said vector at a fixed angular rateabout said longitudinal axis, the length of said vector at any timebeing related to said cross sectional area of said fusible material atany point along said longitudinal axis, said winding having two ends,said fuse structure having two terminals, said terminals being connectedto said ends of said fuse element, the longitudinal components ofdistance between adjacent sections of the winding as measured in a planeincluding said longitudinal axis being substantially equal, said windingbeing supported substantially only at said ends.
 2. A current limitingfuse structure comprising a fuse element including fusible materialhaving the characteristic shape of a helical coil, said fusible materialhaving a generally variable cross-sectional area, said helical coilhaving a central longitudinal axis, a variable radius and two ends, saidradius of said helical coil at any point being related to saidcross-sectional area of said fusible material at that point, thelongitudinal components of distance between correspoonding portions ofsaid helical coil as measured in a plane including said longitudinalaxis being substantially equal, said distance being dependent on theaxial length of said helical coil within a predetermined range of axiallengths, said coil being supported substantially only at said two ends.3. The combination as claimed in claim 2 wherein said cross-sectionalarea of said fuse material varies generally discretely in steps, saidvariable radius correspondingly varying discretely in steps.
 4. Thecombination as claimed in claim 3 where in said cross-sectional area ofsaid fuse material is generally circular.
 5. The combination as claimedin claim 2 wherein said cross-sectional area of said fusible material isgenerally circular, said circular fusible material having a radiuswherein said latter-mentioned radius varies substantially linearly alongthe length of said fuse material, said radius of said helical coilcorrespondingly varying substantially linearly.
 6. A current limitingfue structure comprising a fuse element, electrically conducting endpieces, a dielectric container disposed to enclose said fuse element,said container being disposed between said end pieces, said fuse elementcomprising resilient fusible material having the characteristic shape ofa helical coil, said fusible material having a generally variablecross-sectional area, said helical coil having a variable radius, and alongitudinal central axis, said fusible material having two endsconnected to said electrically conducting end pieces, said radius ofsaid helical coil being related to said cross-sectional area of saidfuse material such that the longitudinal separations between adjacentportions of said helical coil, as measured in a plane containing saidlongitudinal axis, are substantially equal, regardless of the axiallength of said helical coil within predetermined limits, said helicalcoil being supported only at said ends and primarily by said end pieces.7. The combination as claimed in claim 6 wherein said cross-sectionalarea of said fuse material varies generally discretely in steps, saidvariable radius correspoindingly varying discretely in steps.
 8. Thecombination as claimed in claim 7 wherein said cross-sectional area ofsaid fuse material is generally circular.
 9. The combination as claimedin claim 6 wherein said corss-sectional area of said fusible material isgenerally circular, said circular fuse material having a radius whereinsaid latter-mentioned radius varies substantially linearly along thelength of said fuse material, said radius of said helical coilcorrespoindingly varying substantially linearly.
 10. The combination asclaimed in claim 9 wherein pulverulent arc quenching material isdisposed around said fuse element and contained within said dielectriccartridge.